Generally, contact lenses in wide use fall into three categories: (1) hard lenses formed from materials prepared by polymerization of acrylic esters, such as polymethyl methacrylate (PMMA), (2) rigid gas permeable (RGP) lenses formed from silicone acrylates and fluorosilicone methacrylates, and (3) gel, hydrogel or soft type lenses. The hard and rigid-type lenses, because they are characterized by low vapor diffusion and absorb only minor amounts of aqueous fluids, have a lower tendency to bind ingredients used in contact-lens care solutions. On the other hand, soft lenses have a greater tendency to bind active ingredients in contact-lens solutions and, therefore, it is especially challenging to develop solutions designed for the treatment of soft-type lenses, whether made from the more traditional copolymers of 2-hydroxyethylene methacrylate (HEMA) or from the newer silicon-containing hydrogel materials.
In the normal course of wearing contact lenses, tear film and debris consisting of proteinaceous, oily, sebaceous, and related organic matter have a tendency to deposit and build up on lens surfaces. Many factors influence deposit formation, including patient to patient variation, lens material, care regimen, and environment. In general, high water, ionic lens materials absorb more protein than low water or non-ionic lens materials. As part of the routine care regimen, contact lenses must be cleaned to remove these tear film deposits and debris. If these deposits are not properly removed, both the wettability and optical clarity of the lenses are substantially reduced and wearer discomfort may result.
Further, contact lenses must also be disinfected to kill harmful microorganisms that may be present or grow on the lenses. Some of the most popular products for disinfecting lenses are multi-purpose solutions that can be used to clean, disinfect and wet contact lenses, followed by direct insertion (placement on the eye) without rinsing. Obviously, the ability to use a single solution for contact-lens care is an advantage. Such a solution, however, must be particularly gentle to the eye, since at least some of the solution will be on the lens when inserted and will come into contact with the eye.
British Patent No. 1,432,345 discloses contact lens disinfecting compositions containing a polymeric biguanide and a mixed phosphate buffer. Compositions as disclosed by this patent, however, have corneal staining values of 17% or more, far above that which is desirable for patient acceptability.
U.S. Pat. No. 4,758,595 to Ogunbiyi et al. disclosed that a contact-lens solution containing a polyaminopropyl biguanide (PAPB), also known as polyhexamethylene biguanide (PHMB), has enhanced efficacy when combined with a borate buffer. These disinfecting and preservative solutions are especially noteworthy for their broad spectrum of bactericidal and fungicidal activity at low concentrations coupled with very low toxicity when used with soft-type contact lenses. Compositions containing PHMB and borate have been commercialized in various products including multi-purpose solutions, at levels of about 1 ppm or less for use with soft contact lenses.
The fact that multi-purpose solutions are designed for use as a wetting agent, without rinsing, means that the solution must be ophthalmically safe for eye contact. This limits, to some extent, the type and concentration of both cleaning agents and biocides that can be employed in the solution. For example, as can be readily understood, biocides or cleaners in a shampoo product may not be suitable for ophthalmic use. A challenge has been to develop a formula that is, on the one hand, maximally efficacious and, on the other hand, sufficiently gentle to be not only safe, but comfortable for in-the-eye use.
With conventional contact-lens cleaners or disinfectants, including multi-purpose solutions, lens wearers typically need to digitally or manually rub the contact lenses (typically between a finger and palm or between fingers) during treatment of the contact lenses. The necessity for the daily "rubbing" of contact lenses adds to the time and effort involved in the daily care of contact lenses. Many contact-lens wearers dislike having to perform such a regimen or consider it to be an inconvenience. Some wearers may be negligent in the proper "rubbing" regimen, which may result in contact-lens discomfort and other problems. Sometimes rubbing, if performed too rigorously, which is particularly apt to occur with begining lens wearers, may damage the lenses. This can be problematic when a replacement lens is not immediately available.
Contact lens solutions that qualify as a "Chemical Disinfecting Solution" do not require rubbing to meet biocidal performance criteria (for destroying representative bacteria and fungi) set by the US Food and Drug Administration (FDA) under the Premarket Notification (510k) Guidance Document For Contact Lens Care Products, May 1, 1997. In contrast, a contact-lens solution, referred to as a "Chemical Disinfecting System," not qualifiing as a Chemical Disinfecting Solution, requires a rubbing regimen to pass biocidal performance criteria. Traditionally, multi-purpose solutions (used for disinfecting and wetting or for disinfecting, cleaning, and wetting) have qualified as a Chemical Disinfecting System, but not as a Chemical Disinfecting Solution.
Traditional contact-lens solutions may depend on the rubbing regimen, not only for efficacious disinfection, but for efficacious cleaning. Thus, in order to develop a contact-lens care solution that would provide efficacious cleaning without a rubbing regimen for cleaning would require improved cleaning while still being sufficiently gentle for in-the-eye use.
It would be desirable to obtain a multi-purpose contact-lens solution that would provide increased cleaning efficacy. It would be desirable to obtain such improved cleaning efficacy while (1) maintaining the biocidal efficacy of the product and (2) maintaining a low order of toxicity to eye tissue, such that after the solution is used to treat a contact lens, the lens can subsequently be placed on the eye without rinsing the solution from the lens. While still more challenging to develop, it would also be desirable to obtain a solution that exhibits both efficacious cleaning and disinfecting of a contact lens, without requiring a rubbing regimen, or at least not inherently or invariably requiring it for acceptable performance, and which solution would still allow direct placement of the contact lens on an eye following soaking in the solution and/or rinsing and rewetting with the solution.